Batteries are a convenient source of electrical energy for many types of portable and/or mobile electronics. A typical battery is formed by the connection of a number of electrical cells connected in a series configuration. Many types of batteries include rechargeable cells, such that when an outside energy source is applied to the battery cells energy is stored within the cells. While many chemical combinations for the cathode and the anode of the battery cells exist, some commonly used combinations include nickel cadmium (NiCd), nickel metal hydride (NiMH), and lithium ion (Li-Ion) compositions.
While the rechargeable battery cells provide a convenient source of power, the rechargeable battery cells do not have an infinite life span and the ability of the cells to hold a charge degrades over the lifetime of the cell. Furthermore, battery cells leak energy and lose charge during periods of non-use or storage. These periods of non-use may occur while the cells are being held by a manufacturer before the cells are assembled into a battery, or while the cells are assembled into the battery, but the battery has not yet been sold, or during long periods of non-use of the battery by the consumer. The cell leakage is exacerbated by the presence of heat, which may affect batteries that are not stored in a temperature controlled environment. Alternatively, the cell leakage due to heat may affect a battery, such as a back up battery, that is held in close proximity to other normally operating electrical circuits that give off heat. Furthermore, due to the internal resistance of the battery, battery cells may not discharge evenly within the battery. All of these factors cause each battery cell to hold a different level of charge in comparison to the other cells in the battery. These different levels of charge can result in unpredictable indications of low battery and possible inadvertent power shut downs.
Battery monitors are used to monitor the remaining battery discharge time by monitoring both the total charge of all the battery cells and the charge on each individual cell. Typically, a predetermined minimum charge threshold is established for both the total battery charge and for the individual cell charge. If either measure falls below the respective predetermined minimum threshold, the battery terminates discharge, cutting the power to the electronic device. Typically, the individual cell predetermined threshold accounts for possible cell imbalance by allowing an individual cell to operate at a charge level lower than that cell's fractional share of the total battery charge predetermined threshold. For example, if a battery has three cells and the battery cell predetermined minimum charge is 9 volts, the individual cell predetermined minimum charge may be 2.4 volts instead of the 3 volts that would be one third of the 9 volt total. The battery may also produce an indication of the remaining battery charge and this indication is determined by using the total cell charge of all the battery cells. Therefore, if the cells are unbalanced, one cell may fall below the individual cell charge threshold before the total battery charge reaches the total battery charge minimum threshold. This results in the battery terminating the supply of a charge before the total battery charge has reached the minimum threshold value. This reduces the operable battery time between charges and produces an unpredictable termination of the battery discharge.
Similarly when the battery is charging, the cells are charged until the total charge on the cells reaches a threshold value. Additionally, charging is also terminated when the charge on any one individual cell reaches predetermined maximum charge threshold. Therefore, if the cells are unbalanced, one cell may reach the individual cell maximum charge threshold, while the total charge of the battery has not yet achieved full battery charge. This results in the battery not being charged to its full charge potential, which reduces the operable battery time between charges.
Methods and systems have been developed to attempt to remedy the problem of cell unbalance. The manual process of measuring the state of charge of each cell and matching the cells with similar charges when they are assembled into a battery is inaccurate and very time consuming. Furthermore, this manual process does not address any cell leakage that may occur after battery assembly. Alternatively, active cell balancing may be implemented using a series combination of a load resistor and a transistor placed in parallel with each individual cell, where the resistor and transistor shunt current during charge of the battery cells. This type of cell balancing is achieved by software using a complex control algorithm and merely emulates the cell charge balance rather than forcing an inherent cell charge balance and therefore is sensitive to many sources of error, such as initial cell charge and the cell open circuit voltage.
Therefore, it is desirable in the field of rechargeable batteries to provide a battery that is capable of balancing the charge of each of the cells to maximize the operable battery discharge time. It is also desirable for a battery that achieves cell charge balancing that is not as dependant on complex software algorithms to emulate the forcing of inherent cell balance. It is desirable for a battery that achieves cell balancing and uses measured values of cell characteristics in controlling the cell balancing. Furthermore, a method for balancing the charge on the cells of a battery is desirable. It is desirable for the method to be operable prior to assembly of the cells into a battery, or for the method to be operable after the battery has been constructed.